Seismic isolators are known which are provided in a building and hinder vibrations from being transmitted to the side of the building when an earthquake occurs. Among the seismic isolators, for example, as described below, Patent Document 1 shows “an air-bag seismic isolator” which allows a control means, such as a sensor detecting vibrations, to inflate an air bag thereof. Patent Document 2 shows “an air-pressure seismic-isolation positional-shift restorer” which inflates an air sack after an earthquake subsides and thereby restores an upper foundation and a lower foundation from a positional shift thereof. However, the air bag or air sack employed therein operates with the air stored in an air tank, and hence, once the air bag or sack operates by sensing a seismic motion, then it needs refilling with air, thereby taking time. This means that when a great earthquake occurs, the air bag or sack may be unusable against aftershocks following the earthquake. Patent Document 3 shows “a magnetic-levitation seismic isolator” which obtains a seismic-isolation effect by utilizing the repulsion of the magnetic forces of magnets provided in upper and lower foundations. When an earthquake occurs, the seismic isolator senses vibrations caused thereby, and then, leads an electric current to flow between the electromagnets. In addition, among seismic-isolation mechanisms employed for seismic isolators, there are a rolling seismic-isolation bearing, a laminated-rubber bearing, a sliding seismic-isolation bearing and the like. The rolling seismic-isolation bearing is provided with a ball, a roller or the like, and the ball, the roller or the like is borne by a flat or concave plate and rolls thereon. Consequently, the rolling seismic-isolation bearing requires a plate-like bearer. Furthermore, measures to prevent winds from vibrating a seismic isolator need to be regularly taken.